HEATH — About seven hours after battling a house fire at 21 Route 8A, nine fire departments returned to battle the blaze they thought they had vanquished but that had rekindled, ultimately destroying the ranch house of David W. Kratt.

“We don’t know what caused the rekindling. We thought it was out when we left,” Heath Deputy Fire Chief Ken Gilbert said on Thursday.

Gilbert said that a passer-by reported the second fire at 9:24 p.m. Wednesday. “When we returned,” he said, “it was fully engulfed in flames.”

“I don’t know how it re-ignited,” he said. The single-floor house, which had a metal roof, was left standing after the first fire, although the interior was essentially reduced to charred debris. After the second fire, nothing was left standing.

As of Thursday afternoon, Gilbert said he had not yet been able to reach Kratt to talk to him about the fire.

Jennifer Meith, a spokeswoman for the state Department of Fire Services, said the state Fire Marshal’s Office is continuing to investigate the cause of the first fire, which happened around 2 p.m. Wednesday.

When asked if arson was suspected, Mieth didn’t answer directly. “We’re following our usual protocol, looking for where the fire started,” she said.

“Rekindles happen all the time,” she said of the nighttime fire. She said firefighters do their best to make sure everything has been extinguished, but it’s not always possible to find every potential danger.

Of the second fire, she said, “There was a lot of destruction; there’s not a lot for investigators to work on.”